This invention relates to polypropylene homopolymers, copolymers and their blends stabilized against oxidative degradation with a combination of at least one diarylamine derivative as hereinafter defined and at least one sterically hindered phenol.
A long-standing problem of plastic goods manufacturers is stabilizing polypropylene resins during high temperature, high shear blending and extrusion processes which are common in plastic goods manufacturing. It has been found that resins that show excellent long-term thermal heat stability characteristics may not be sufficiently protected against the type of degradation which occurs in an extruder during long residence times. The diphenylamine/phenolic blends of this invention have been found to unexpectedly protect the polypropylene against both polymer breakdown, as shown by superior melt flow stability, and against the formation of undesirable color bodies as shown by low color index values.
Aralkyl-substituted diarylamines such as 4,4'-bis(alpha,alpha-dimethylbenzyl)diphenylamine (NAUGARD 445, Uniroyal Chemical) and their use as antioxidants for a variety of polymeric materials are known from U.S. Pat. Nos. 3,452,056 and 3,505,225.
Sterically hindered phenols constitute another known class of antioxidant materials. Antioxidant compositions containing both an amine component and a sterically hindered phenol component, with and without other ingredients, are also known.
Thus, U.S. Pat. No. 3,304,283 discloses an antioxidant composition for mono-olefinic polymers such as polypropylene containing at least one aromatic phenolic thioether, diaryl thioether, aliphatic disulfide, aromatic disulfide, aromatic mercaptan, aliphatic mercaptan and/or aliphatic thiuramdisulfide in combination with at least one biphenol and/or aromatic amine.
U.S. Pat. No. 3,432,578 discloses the stabilization of conjugated diene polymers from the adverse effects of ultraviolet radiation employing a mixture of a diaryl hydroxylamine, 2,6-di-tert-butyl-4-methylphenol, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4 hydroxylbenzyl)benzene, dilaurylthiodipropionate and lignin sulfonate.
U.S. Pat. Nos. 3,567,664 and 3,637,865 each describes the stabilization of polyether polyol-based polyurethanes against scorching employing a mixture of 2,6-ditertiary-butyl-4-methyl phenol and a p,p'-dialkyl diphenylamine. Similarly, U.S. Pat. No. 4,007,230 describes the stabilization of polyether polyol-based polyurethanes against scorching employing a composition consisting of certain hindered phenols such as 2,4-di-methyl-6-octyl phenol and 4,4'-bis(alpha,alpha-dimethylbenzyl)diphenylamine (i.e., NAUGARD 445, Uniroyal Chemical, referred to supra).
U.S. Pat. No. 3,655,559 discloses stabilizers for synthetic lubricants based on alkylated diphenylamines and, optionally, any one of numerous other kinds of antioxidants, sterically hindered phenols amongst them.
U.S. Pat. No. 3,979,180 describes the stabilization of low density polyethylene and mineral-filled ethylene vinyl acetate copolymers employing a combination of a sterically hindered phenol and/or a substituted diphenylamine of the structure Ar-NH-Ar' where Ar and Ar' are each phenyl, naphthol, substituted phenyl and naphthol including alkyl substituents of 1-20 alkyl carbon atoms, and halogen, a metal organic thiophosphorus compound and a trace amount of a transition metal salt.
U.S. Pat. No. 4,341,677 describes oil-in-water emulsions of antioxidants useful for treating fibrous reinforcements such as glass fibers to increase the protection of such fiber reinforced polymeric materials as polyolefins, polyurethanes, polyamides, polyesters, polycarbonates, polyacetals, polystyrene and styrene copolymers against chemical degradation. The emulsions are based on hindered phenols such as octadecyl 3-(3',5'-di-tert-butyl-4-hydroxyphenyl-propionate (IRGANOX 1076, Ciba Geigy) and/or diarylamines such as 4,4'-[2-(2-phenyl)-propyl]diphenylamine (NAUGARD 445, Uniroyal Chemical, referred to supra). All of the specific examples of emulsions disclosed to be useful in U.S. Pat. No. 4,341,677 are based either on a sterically hindered phenol or a diarylamine. No example of an emulsion containing both a hindered phenol and a diarylamine is provided.
U.S. Pat. No. 4,420,579 describes antioxidant compositions for polyolefins based on coordination complexes of nickel thiobis(alkylphenolates) with hydroxy-substituted ligands in combination with co-additives such as diarylamines and/or hindered phenols.
U.S. Pat. No. 4,440,671 describes mixtures of hydrocarbon-substituted diphenylamines, e.g., a liquid diphenylamine alkylated with styrene (WINGSTAY 29, Goodyear Tire and Rubber Company) and high molecular weight polyethylene glycols as water-tree retardant compositions for polyethylenes. The compositions can optionally contain antioxidants such as sterically hindered phenols and amines, polymerized 2,2,4-tetramethylhydroquinoline, 4'4'-thio-bis-(6-tert-butyl-3-methylphenol), thiodiethylenebis-(3,5-ditert-butyl-4-hydroxy hydrocinnamate, distearylthiodipropionate, and the like.
The inventor's copending application Ser. No. 941,479, filed Dec. 11, 1986, now abandoned and refiled as Ser. No. 188,149 filed Apr. 27, 1988 discloses mixtures of aralkyl-substituted diarylamines with sterically hindered phenols for stabilization of polyolefins. Specifically preferred is an alpha-methyl styrene substituted diphenylamine, 4,4'-bis(alpha,alpha'dimethylbenzyl)diphenylamine.